Nuclear bodies (NBs) are ubiquitous membrane-less structures that play important but poorly-understood roles in gene regulation. NBs locally increase the concentration of molecules involved in chromatin remodeling, transcription initiation, and RNA processing. Despite their functional importance, and decades of study, we lack a quantitative, mechanistic understanding of NB assembly. Understanding the biophysical rules governing NB assembly and properties is key to elucidating their function. Our group has pioneered the concept that NBs are liquid phase droplets that assemble through phase transitions. Here we will build on this framework, and test it, by developing a new technology that uses light to control nucleoplasmic phase transitions. This technology will enable precise spatiotemporal control of the assembly of NBs and their viscoelastic properties, as well as testing the impact on composition, function, and genome architecture. Our team is uniquely positioned to develop this exciting technology and exploit it to study NBs, both in these Specific Aims, as well as together in future collaborations within the 4D Nucleome Program.